1. Field of the Invention
The present invention relates to methods of making large-volume CaF2 single crystals, which have reduced scattering and improved laser stability, which can be tempered at elevated temperatures and which have an especially great uniformity and low small angle grain boundaries and stress birefringence. The present invention also concerns the single crystals made by the method and their uses.
2. Related Art
To manufacture electronic computer components, such as computer chips and other integrated circuits, by optical lithography the structures for those circuits are imaged by means of a photomask on a wafer and it is irradiated. The need for more powerful computers results in a demand for even smaller computer chips. This makes it necessary to produce ever-sharper images of the respective circuit structures. This necessarily also leads to the use of ever-smaller wavelengths. In the meantime the images are even thinner and smaller than the wavelengths of the respective light employed. Thus laser light of wavelengths from 248 to 365 nm, in the meantime, wavelengths of 157 nm to 193 nm, are used in order to image the circuit structures as sharply as possible on the corresponding wafer. Furthermore additional imaging techniques were developed, which permit sharp imaging of those structures, whose lines are smaller than the wavelengths of the light employed. These additional imaging techniques include especially the so-called technologies for improving resolution (Resolution enhancement technology, RET), for example OPC (optical proximity technology), the use of different off-axis illumination techniques, the so-called phase shift masks and double exposure systems. However these high performance technologies are based on the assumption that the optical lens systems and the optical materials are of higher quality. In this connection CaF2 single crystals have attained increasing importance in comparison to synthetic quartz glass as lens material. In fact at working wavelengths of 157 nm only CaF2 can be used, since quartz glass is no longer permeable for light of this wavelength. Up to now large CaF2 single crystals were used in irradiating optics, i.e. the optics for irradiating the photomask.
It is generally known that optical properties of a single crystal depend entirely on the quality of the crystal. Single crystals of this sort are real crystals, i.e. crystals with real lattices that deviate from the theoretical ideal structure and include structural defects. These types of crystal defects e.g. are produced when individual lattice sites are not occupied during growth, which leads to the so-called defect locations, such as point defects, or also to dislocations like linear defects. However faults in the lattice structure are also produced by the unavoidable impurities present, including foreign ions.
The quality, i.e. the perfection of a crystal, depends on the selected growth method and the manner in which it is performed, i.e. especially on the parameters of the method, like the growth rate and the temperatures during growth and cooling. Thus it has been shown that by tempering the crystal, i.e. by heating an already grown crystal over a long time interval at high temperatures the few structural defects, such as small angle grain boundaries and stress birefringence produced by different lattice stress can be dramatically reduced. However it has also been shown that the light scattering in the crystals is greatly increased by long tempering of this sort. Indeed extended tempering increases both the scattering of reflected light and also the scattering of transmitted light. The scattering of reflected light is described by the so-called bidirectional reflection distribution function, BDRF, and the scattering of the light passing through the crystal is described by the so-called bidirectional transmission distribution function, BTDF. Both types of scattering may be combined in the so-called bi-directional scattering distribution function, BSDF. This BSDF is used for quantitative description of the scattering behavior of a sample. For perpendicular incident light it is a function of the scattered light angle θs and the scattered light azimuth angle φs. It describes the behavior of the measured light scattering power Ps in a solid angle, dΩs, given by the measuring apparatus in relation to the incident power Pi. According to Stover (see J. C. Stover, Optical Scattering—Measurement and Analysis, McGraw-Hill Inc., 1990) it is defined byBSDF=(Ps/Ωs)/(Pi cos θs).
The cosine factor projects the irradiated scattering volume on the direction of the scattering angle θs and thus allows a directional comparison to light scattering measurements on surfaces. The units of BSDF are 1/stearadian. The power of the scattered light is detected under a solid scattering angle θs for characterizing the light scattering behavior of transparent samples, so that the BSDF value is given for θs=constant as a quantifiable light scattering parameter.
A crystal with light scattering that is already observable with the eye, even if it is comparatively small is not usable or only usable in a limited manner for electronic lithography, especially for image formation optics. Both diffusion scattering and also discrete scattering are troublesome.
This type of crystals may be made in many ways known to those skilled in the art, for example by the most frequently used methods such as the gradient solidification methods (GSM methods) and the Bridgman-Stockbarger method.
R. Leckebusch and K. Recker, in Journal of Crystal Growth 13/14, pp. 276-281 (1972) describe an experiment regarding the perfecting of the manufacture of CaF2 single crystals depending on the crystal growth technique. They report that in vapor-grown crystals, in which evaporated crystal material is deposited on a seed crystal, only a very small number of dislocations were observed on the (111)-cleavage plane because of extremely small growth rates and small temperature gradients within the growing crystal under process conditions in which few impurities were present in the crystal. However this process could produce only small crystals of a few mm size because of the very small growth rates. These small volume crystals were not suitable for use in photolithography. For photolithography crystals with diameters of at least 20 or 30 cm and heights of at least 10 or 20 cm are required. Currently there is a need for single crystals of even significantly larger volume.
Also a large amount of already processed, but not further useable, CaF2 crystals is wasted because of cutting processes and optical properties, which are unsuitable for lithographic applications, such as crystal structural defects, poor homogeneity, large birefringence and poor transmission and/or to large scattering. This waste must be disposed of in a troublesome and cost intensive manner.